1. Field of the Invention
This invention relates to valves, and concerns in particular the mechanism by which ball valves--especially those employed in tools used for the testing of subterranean wells, particularly oil wells--are operated against severe resistive forces.
2. Description of the Prior Art
Whether at sea or on land, the first steps in the production of a new hydrocarbon well--an oil well--are the drilling of the well bore itself through the various formations within the earth's crust beneath the drilling rig, followed by "casing" (the introduction and cementing into position of piping which will serve to support and line the bore) and the placing in the bore, at the depth of a formation of interest, of a device known as a packer, into which inner tubing (of smaller diameter than the casing) can subsequently be lodged.
The next work carried out is normally some programme of testing, for the purpose of evaluating the production potential of the chosen formation. The testing procedure usually involves the measurement of downhole temperatures and pressures, in both static and flow conditions (the latter being when fluid from the relevant formation is allowed to flow into and up the well), and the subsequent calculation of various well parameters. To collect the necessary data there is lowered into the well a test string--a length of tubing containing the tools required for testing. The flow of fluid from the formation of interest into the well bore and thus to the test tools is controlled by a valve known as a sub-surface control valve, and it is with valves suitable for this purpose, and their operating mechanisms, that the invention is concerned.
The operation of the various tools included in the downhole test string can be effected using one of three main types of mechanism. These types are those actuated by reciprocal motion of the pipe string (the inner tube, of which the test string constitutes a part), by rotational motion of the pipe string, or by changes in the pressure differential between the tubing and the annular space which surrounds it in the well--hereinafter referred to simply as "the annulus". Test strings wherein the tools thereof are actuated by changes in annulus pressure are at present much in vogue, and it is this type of actuation mechanism that is to be employed with the valves of the invention.
A mechanism of the annulus pressure-responsive type requires the provision and maintenance of a fixed "reference" pressure within the tool. This, used in conjunction with an adjustable (and higher) annulus pressure, allows the establishment of the chosen pressure differential necessary to control the operation of the appropriate component of the test string. The achievement of such a fixed reference pressure is the subject of our co-pending British Patent Application No. 89/07,098.1.
An essential component of the test string is a valve known as the sub-surface control valve. This governs the overall control of the testing procedure by permitting the flow of fluid out of the formation and into and up the well tubing to the various test tools. The density of drilling fluid in the tubing above this valve is adjusted such that its hydrostatic pressure at the depth of the formation is lower than the formation fluid pressure. Thus, when the valve is opened, formation fluid is permitted to enter the well bore through perforations in the casing and flow into the tubing string (and possibly to the surface therethrough). This contrasts with the situation during drilling, when the drilling fluid must exert a hydrostatic pressure greater than the formation fluid pressure in order to prevent the fluid's escape to the surface.
The sub-surface control valve is conveniently an example of a type of valve known as a ball valve. In principle, a ball valve is a variety of valve in which the valve member--the part physically preventing or allowing the passage of some fluid material along a passageway (a pipe, say)--is shaped like a ball with a hole extending therethrough, and is mounted in the passageway for rotation between a position where the hole is aligned with the passageway, so opening the valve and allowing fluid to pass therethrough, and another position where the hole is not so aligned, so closing the valve and preventing fluid passing therethrough. The ball may be positively driven in both directions--that is to say, it may be necessary to apply an actuating force to the ball to open the valve, and then to apply another, and opposite, actuating force to close the valve. An alternative, however, is to bias the ball in one sense--valve closed, say--so that it is only necessary to apply a positive actuating force when the ball is to be moved in the opposite sense--the valve is to be opened, say--and merely removing this force allows the bias to return the ball to its original position (thus, to close the valve).
The manner in which the ball is caused to rotate may employ any one of several different mechanisms, but a convenient one uses a pin projecting from the ball at a position spaced from the intended rotational axis, so that it acts like a crank, and a force applied to the pin round the pin's and the axis' common plane will rotate the ball.
The mounting of the ball will generally be in some sort of sealing seating arrangement (so that fluid cannot easily pass around the ball valve in the unaligned position), and often the ball will also be positively pivoted on some axle-like members, so that it rotates around an axis defined by these members. In some situations, however, it is possible and/or desirable to do without the pivot axle, and to have the ball float--sit "loosely"--within its mounting, restrained only by the seating and the passageway's side walls. This arrangement can work well, but nevertheless does give rise to problems under certain circumstances. More specifically, where the pressure of fluid acting on the upstream side of the ball is very high, the frictional forces caused by the ball being pressed up against the seating on its downstream side may also be high--indeed, they may be so high as to necessitate the application of an inordinately large force to rotate the ball thereagainst into its aligned, valve-open, position. It may even be that the rotational force required to be applied is higher than is available. It is this problem with which the invention is concerned.
A typical example of the thus-described "floating ball" ball valve is that commonly used in an oil well's test string sub-surface control valve. This valve may be actuated in a number of ways, but conveniently there is employed a modified version of the projecting pin arrangement mentioned above, wherein a fixed pin on a pipe-aligned actuating sleeve (or "mandrel") moveable back and forth "along" the pipe projects into an off-axis slot on the ball's side face, so that moving the sleeve provides, via the pin, the required normal force, and so rotates the ball. As discussed above, the operation of the various components in the test string is initiated and driven by applied annulus pressure.
An oil well, however, is a prime example of an environment in which the pressure of the fluid on the upstream side of the valve may be very high. Thus, where (as is usually the case) the ball valve is of the "floating ball" variety the ball (the valve member) is pressed hard up against the seating on the downstream side--so hard, perhaps, that the frictional forces involved are sufficient to prevent the ball rotating when the valve-opening force is applied. If such a situation is indeed encountered in a case where the valve is the sub-surface control valve for, say, a test pipe string, then there may be nothing for it but to remove the string and replace the valve.
This is extremely time-consuming, and so expensive, and the invention seeks to avoid the problem by enabling a much higher actuating force to be applied to the ball, high enough to overcome any foreseeable frictional forces. More particularly, the invention suggests that the actuating force be applied to the valve via a differential thread force multiplier, so that through the same, relatively low actuating force can be applied the ball experiences a much higher force--and thus opens despite the fluid-pressure-induced friction.
In an arrangement in which the valve is biased closed (in order that in the event of an accident it may "fail safe"), and is operated by the application to the annulus of a pressure that is high relative to the tool-contained reference pressure--an annulus over-pressure--it is necessary to continue to apply this annulus pressure in order to keep the valve open. It may be inconvenient, though, to maintain the application of the pressure for extended periods. Nevertheless, the well testing procedure may require that the sub-surface control valve remain open for such extended periods--and, obviously, these considerations are mutually exclusive. In principle, this dilemma can be solved, for a valve having a ball within a seating and an associated actuating sleeve biased closed, by providing latch means whereby the valve actuating sleeve may be retained in the valve-open position even when the applied annulus pressure is removed. However, there is then naturally required some means of eventually unlatching the sleeve, and so closing the valve. The invention suggests just such a solution wherein a first pulse of applied annulus pressure causes the valve to open and then, as the pressure is removed, become latched in the open state, and thereafter a second pulse of applied annulus pressure unlatches the valve and allows it to close once the pulse has ended.